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OH + HONO reaction: A theoretical study
Journal of Molecular Structure: THEOCHEM 847 (2007) 10–22 www.elsevier.com/locate/theochem
OH + HONO reaction: A theoretical study De-Quan Wang, Ji-Lai Li, Xu-Ri Huang *, Cai-Yun Geng, Chia-Chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China Received 19 June 2007; received in revised form 24 August 2007; accepted 24 August 2007 Available online 11 September 2007
Abstract The reaction of OH radical with nitrous acid HONO is investigated by ab initio quantum chemistry methods. The doublet potential energy surfaces are calculated at the CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) levels. Various possible reaction pathways are considered. Among them, the most feasible pathway should be the OH radical attacking on the hydrogen of cis-HONO to form a 6-member-ring complex C2 barrierlessly, followed by the indirect hydrogen abstraction transition state TSabsC2-C6I to form a weakly bound complex C6, giving rise to the educts P1 H2O + NO2. Because all of the complexes, transition state, and products involved in the feasible pathway lie below the reactants, the title reaction is expected to be rapid, which in good agreement experiment. The present study may be helpful for probing the mechanisms of the HONO reactions and understanding the atmospheric chemistry. 2007 Elsevier B.V. All rights reserved. Keywords: Reaction mechanism; Potential energy surface; OH radical; HONO
1. Introduction Radical-molecule reactions play important roles in both combustion and interstellar processes [1]. Reactions with small or even zero barriers are of particular interest [2–6], especially in interstellar space where the temperature is low. Under these conditions, low-barrier reactions easily deplete molecules and yield new products. The reaction of hydroxyl radical with nitrous acid is very important to the chemistry of troposphere where HONO may be formed by lighting or chemical reactions in the polluted environment. Surprisingly, data on non-reactive uptake of nitrous acid on ice are scarce, given the importance of HONO for atmospheric and especially polar snow chemistry [7–10]. On the other hand, the reaction is also pivotal to our understanding of high-temperature combustion of nitrate esters, nitramines and energetic materials containing [H, N, O]-species [11]. It has been long known that nitrous acid is a key reactive intermediate during the course of chemical
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0166-1280/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.08.037
combustion reactions of energetic materials such as nitramines propellants [12–15]. On account of its relevancy in combustion and atmospheric chemistry, kinetic and mechanistic information on H, O, NO2, O3, HCl, F, HF, HNO and NH3 initiated reaction with nitrous acid has been subject to numerous experimental and theoretical studies [16– 21]. The most important chemical cleaning agent of the atmosphere is the hydroxyl radical, OH [22,23]. It determines the oxidizing power of the atmosphere, and thereby controls the removal of nearly all gaseous atmospheric pollutants [24,25]. The atmospheric supply of OH is limited, however, and could be overcome by consumption due to increasing pollution and climate change [26,27], with detrimental feedback effects. Recently Rohrer etc. reported that the concentration of OH is linear dependence on solar ultraviolet radiation [28]. Literaturely, there have been limited but conflicting kinetic data [29–38] for the OH + HONO reaction, which has been invariably assumed to occur by the abstraction reaction: OH þ HNO2 ! H2 O þ NO2
DH ¼ 41 kcal=mol
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In the temperature range of interest to atmospheric chemistry, Cox and co-workers reported the rate constant at ambient temperature to fall within the range of (1– 4) · 1012 cm3 mol1 s1 with small positive activation energy, 3.2 kJ/mol, and independent of pressure [30–34]. Recently, Burkholder et al. [35] determined the rate constant by LIF (laser-induced fluorescence) for OH detection and reported k1 = (2.8 ± 1.3) · 1012 exp [(+260 ± 140)/ T] cm3 mol1 s1 for the temperature range 298–373 K. The activation energy for the reaction was found to be 2.2 kJ/mol, clearly at odds with that of Jenkin and Cox mentioned above. Furthermore, another two conflicting rate constant expressions have been recommended: k1 = 3.76 · 1012 T1.0 exp (68/T) cm3 mol1 s1 for 300– 2500 K by Tsang and Herron [36] for high-temperature combustion modeling applications, k1 = 1.63 · 1012 exp (+260/T) cm3 mol1 s1 at 290–380 K by Atkinson et al. [37] for atmospheric modeling applications. On the other hand, a theoretical study [11a] on the title reaction was carried out, and the equilibrium geometries and transition structures were calculated using density functional theory (DFT) method. As we know, different in geometry will lead to different reaction mechanisms and extreme care should be taken to choose an appropriate theoretical level for investigation of reactions involving radicals [39]. Thus, it is necessary to reexamine the title reaction with other theoretical method to provide theoretical knowledge accurate enough to be useful for further kinetic and thermodynamic study on radical reactions. Some conclusions that are made in this work may be helpful for further theoretical and experimental studies of this title reaction. In the title reaction there are pre-reactive complexes. Six pre-reactive complexes are successfully located on the doublet PESs. All the six structures with no imaginary frequencies (minima) were located at all levels of theory. As shown in Fig. 1, most of the addition and the H-abstraction reactions proceed via complexes in the title reaction. Another theoretical investigation of the isoelectronic system C2H2 + O [40], shows that a collinear hydrogenbonded complex C2H OH (productlike) exist on the PES, which is viewed as a direct precursor for the proton transfer. While in another isoelectronic system C2H2 + OH, there is no pre-reactive complex located on the H-abstraction path, but on the addition path [41]. On the other hand, all the ab initio calculations show that the approach of H/O atom of OH radical to HONO generates six different initial pre-reactive complexes, depending on the details of the approach geometry. 2. Computational methods All the calculations were carried out on the SGI O3800 servers using the GAUSSIAN 98 and GAUSSIAN 03 program package [42]. Geometries of the reactants, products, intermediates and transition states (TSs) have been optimized with the unrestricted Møller–Plesset secondorder perturbation UMP2 (FULL) [43] method using
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the 6-311G(d,p), and 6-311++G(d,p) basis sets. The presence of diffuse functions in the basis set allows for an appropriate representation of the dispersion forces that should play an important role in the stabilization of the weakly bound structures considered in this work [44]. Vibrational frequencies, also calculated at the same level of theory, have been used to characterize stationary points, zero-point energy (ZPE) corrections calculations. The number of imaginary frequencies for intermediates and transition states are 0 and 1, respectively. The ZPE and vibrational frequencies were scaled by a factor of 0.95 for anharmonicity correction [45]. To confirm that the transition states connect between designated intermediates, intrinsic reaction coordinate (IRC) [46] calculations were performed at the UMP2/6-311++G(d,p) level of theory. For the purpose of obtaining more reliable energies of various structures, the coupled-cluster CCSD(T) method with Dunning’s correlation-consistent augmented aug-ccpVDZ basis set [47,48] was used. The UMP2/6311++G(d,p) optimized geometries were used for the single-point coupled cluster calculations without reoptimization at the CCSD(T)/aug-cc-pVDZ. The major problem in the application of unrestricted single determinant reference wave function is that of contamination with higher spin states. The severe spin contamination could lead to a deteriorated estimation of the barrier height [49,50]. We have examined the spin contamination before and after annihilation for the radical species and transition states involved in the OH + HONO reaction. For doublet systems, the expectation values of ÆS2æ after annihilation range from 0.75 to 0.80 (the exact value for a pure doublet is 0.75) except for one complex and six unimportant transition states on the potential energy surface (0.80–0.87). This suggests that the wave function is not severely contaminated by states of higher multiplicity [51,52].Therefore, we expect that the CCSD(T)//MP2 calculations reported in this work are, in this regard, reliable. The thermodynamic functions (DH and DG) are estimated within the ideal gas, rigid-rotor, and harmonic oscillator approximations. A temperature of 298.15 K and a pressure of 1 atm were assumed. 3. Results and discussion For the present H2NO3 system, there are enantiomorphs. For convenient and clear discussion, we only present moieties of reaction channels that have enantiomorphic varieties and some important nantiomorphous structures. The structures of the reactants, complexes, intermediate isomers, transition states and products are depicted in Fig. 1. By means of the interrelation among the reactants, isomers, transition states, and products as well as the corresponding relative energies, the schematic profiles of the doublet potential energy surfaces are depicted as shown in Figs. 2, 3 and Fig. S3. The calculated
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Fig. 1. Optimized geometries of the reactants, the complexes, the doublet intermediate isomers, the products and the transition states for the OH + HONO reaction. Bond distances are in angstroms, and angles are in degrees. Numbers in roman type show the structures at the UMP2/6311++G(d,p) level of theory. Numbers in italics show the structures at the UMP2/6-311G(d,p) level of theory. Numbers in bold show the structures at the Max{CCSD(T)}//IRC{UMP2} + ZPE level of theory. Symbols with asterisks indicate enantiomorphic varieties corresponding to the same symbols.
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Fig. 1 (continued)
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Fig. 1 (continued)
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Fig. 1 (continued)
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Fig. 1 (continued)
energetic data (DE, DH, and DG) of various complexes, isomers, transition states and products are listed in Table S1. For convenient discussion, the energy of OH + cis-HONO is set at zero as a reference for other species. 3.1. Analysis of reaction mechanism of OH radical with HONO As the OH + HONO reaction can have either an abstraction or an addition mechanism, three distinguishable types of initial attacks have been revealed for the radical-molecule reaction OH + HONO, namely, the picking-up on hydrogen (direct/indirect hydrogen abstraction), hydroxyl (direction hydroxyl abstraction), and the attack on the O-atom and
N-atom of HONO (addition–isomerization–elimination). A detailed discussion on the title reaction mechanism is given as follows. 3.1.1. Hydrogen abstraction reaction mechanism Three kinds of hydrogen abstraction reactions can take place in the title reaction, viz. the oxygen of the OH radical picking-up hydrogen of HONO directly or indirectly (single-barrier and double-barriers), and the O/N-atom of the HONO indirect picking-up hydrogen of OH radical. 3.1.1.1. The oxygen of OH radical attack on the hydrogen of HONO. Indirect hydrogen abstraction reaction mechanism I
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Fig. 2. Schematic potential energy surfaces of the hydrogen abstraction channels in the OH + HONO reaction. Numbers in roman type are relative energies at the CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) + ZPE level of theory. Relative energy of complex C7 is calculated at the CCSD(T)/augcc-pVDZ//UMP2/6-311G(d,p) + ZPE level of theory.
(single-barrier hydrogen abstraction). The first step of this mechanism involves the O atom of OH radical indirectly attacking the H-atom of HONO. For cis-HONO, there are three pathways of the indirect hydrogen abstraction reactions, and for trans-HONO, two. These pathways can be expressed as follows: Path1 RP1 (1): R (cis-HONO + OH) fi C2 fi TSabsC2-C6I fi C6 fi P1 NO2 + H2O Path2 RP1 (2): R fi C2 fi TSabsC2- C6II fi C6 fi P1
Path3 RR (1): R fi C2 fi TSabsC2- C2 0 fi C2 0 fi R Path4 R 0 P1 (1): R 0 (OH + trans-HONO) fi C1 fi TSabsC1- C6I fi C6 fi P1 Path5 R 0 P1 (2): R 0 fi C1 fi TSabsC1- C6II fi C6 fi P1 As shown in Fig. 2, the first two indirect hydrogen abstraction processes start from the OH radical attacking on the hydrogen of cis-HONO, forming a 6-member-ring complex C2 (3.94 kcal/mol) barrierlessly. And then C2 can transform to a weakly bound complex C6 (34.32)
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Fig. 3. Selected schematic potential energy surfaces of the associative reaction mechanism, and the important addition–isomerization–elimination channels in the OH + HONO reaction. Numbers in roman type are relative energies at the CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) + ZPE level of theory. Numbers in square brackets are values at the Max-{CCSD(T)}//IRC{UMP2} + ZPE level of theory.
via two H-atom abstraction transition states, TSabsC2-C6I (2.07) and TSabsC2-C6II (1.32), respectively. Subsequently, C6 transforms to the educts P1 H2O + NO2. Since all of the complexes, transition state and products lie below the reactants, the rate of this pathway should be very fast even in low-temperature ranges. On the other hand, considering that the rate-determining transition state TSabsC2C6II lies 1.32 kcal/mol above the reactants in Path2, we can safely draw the conclusion that the reaction pathway Path2 is also feasible on the doublet potential energy surface at room-temperature. However, the higher rate-determining energy barrier in Path2 makes Path2 less competitive than Path1.
Pathway Path3 is indeed an H-atom exchange process between OH radical and cis-HONO. In pathway Path3, complex C2 and C2 0 can be formed barrierless with the association of hydroxyl and cis-HONO. Complex C2 (C2 0 ) can transform to C2 0 (C2) via TSabsC2-C2 0 , with an 18.14 kcal/mol energy barrier. Both complexes, C2 and C2 0 located by IRC calculations, are essentially the mirror image of each other, having 3.94 kcal/mol binding energies with Cs symmetry. Furthermore, a similar exchange process involving the trans-HONO does not exist. The OH radical and trans-HONO can also form a 5member-ring complex C1 in pathways Path4 and Path5.
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Complex C1 has a slightly larger binding energy (4.56) than its cis-counterpart C2. For products formation, however, C1 has to overcome energy barriers TSabsC1-C6I (1.67 kcal/mol), or TSabsC1-C6II (15.40 kcal/mol) to reach C6, in Path4 and Path5, respectively. Thus the indirect hydrogen abstraction process of trans-HONO by OH may proceed through Path4. Indirect hydrogen abstraction reaction mechanism II (double-barriers hydrogen abstraction). There exists a remarkable difference between the trans and cis reaction processes, that is, for the OH radical picking-up the Hatom of trans-HONO process, there is a double-barriers hydrogen abstraction reaction mechanism, viz. indirect hydrogen abstraction reaction mechanism II. To our knowledge, it is the first time that the existence of double barriers in hydrogen abstraction reactions has been reported. The pathway Path7 takes place via the complex C1, transition state TSabsC1-I0, isomer I0, transition state TSabsI0-C7 and the complex C7, giving rise to the separated H2O + NO2 products with the energies of 4.56, 3.56, 4.04, 3.18 and 34.54 kcal/mol, respectively. As shown in Fig. 2, the rate-determining transition state TSabsI0-C7 in Path6 lies somewhat higher than that of Path1, Path2 and Path4, which leads us to the fact that the Path6 is less competitive than those three pathways, but it could also occur at room-temperature. It can be expressed as follows: Path6 R 0 P1 (3): R 0 fi C1 fi TSabsC1- I0 fi I0 fi TSabsI0-C7 fi C7 fi P1 Direct hydrogen abstraction reaction mechanism. Among all the OH radical attack on the H-atom of HONO pathways, only one direct hydrogen abstraction reaction path is revealed in the present investigation. We express it below for discussion concision: Path7 RP1 (3): R fi TSabsR-C7 fi C7 fi P1 In this pathway, the rate-determining hydrogen abstract transition state TSabsR-C7 lies 23.19 kcal/mol higher than the reactants, the rate of this pathway should be very slow at moderate conditions. Consequently, the large barrier also prevents it from competing effectively with the reaction pathways Path1 and Path2. We should mention herein that we could not find the complex C7 at the UMP2/6311++G(d,p) level. To rationalize this situation, we turn to the flatness of the PES in the regions of larger intermolecular distances as can be seen from Fig. 1. To briefly summarize, the pathway Path1 is the most competitive pathway among all of the seven pathways. The pathway Path2, Path4 and path6 may be less competitive dynamically and play a part role at room temperature. 3.1.1.2. The hydrogen of OH radical attack on the O/N-atom of HONO. Our results show that there are additional three hydrogen abstraction reaction channels, namely, the
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hydrogen of OH radical attacking on hydroxyl-oxygen, and nitrogen-atom of HONO. They are listed as follows: Path8 RP1 (4): R fi C4 fi TSabsC4-C6 fi C6 fi P1 Path9 R 0 P1 (5): R 0 fi C3 fi TSabsC3-C6 fi C6 fi P1 Path10 RP8 (4): R fi C5 fi TSabsC5-P8 fi P8 HON(H)O + O As shown in Fig. 1, transition states TSabsC4-C6 (43.05) and TSabsC3-C6 (40.39) possess similar 4-member-ring structures in Path8 and Path9. In these two reaction pathways, IRC calculations show that, the hydrogen of OH radical approaches the hydroxyl-oxygen of HONO firstly, which makes the bond of HO NO broken, and then, the O H bond of OH radical ruptured. Secondly, the O-atom attacks the N-atom of NO, forming the complex C6, leading to P1 eventually. On the other hand, the hydrogen of OH radical can also attack on the nitrogen of cis-HONO, leading to the production of P8 HON(H)O + O through Path10. Since the rate-determining energy barrier in Path10 is considerably high (83.09), we can safely rule it out from the doublet PES in low-temperature ranges. However, in combustion environment, where the temperature is very high (>1000 K), pathways Path8 and Path9, may play an important role. 3.1.2. Associative reaction mechanism There are three reaction channels in the associative reaction mechanism, that is, Path11 13, in which three associative transition structures are successfully established. The paths are listed below: Path11 R 0 P2 (1): R 0 fi TSR 0 -P2 fi P2 H2O2 + NO Path12 RP2 (1): R fi TSR-P2 fi P2 Path13 RR 0 (3): R fi C4 fi TSC4-C1 0 fi C1 0 fi R 0 As shown in Fig. 1, the main feature of the transition state TSR 0 -P2 in the path11 is: It has an imaginary frequency of 228i cm1 and the animation of imaginary frequency corresponds to the movement of OH and NO group, when OH moves toward the O-atom of hydroxyl in trans-HONO, the NO group would be away from it, that is to say, the formation of the O–O bond is concomitant with the breaking of the O–N bond. A corollary, the formation of the O–N bond would be concomitant with the breaking of the O–O bond. The transition states TSR-P2 and TSC4-C1 0 own the similar feature as well except the different moving groups. That is, TSR-P2 corresponds to the movement of OH radical and NO in cis-HONO, which is reported for the first time, while TSC4-C1 0 corresponds to the movement of the OH radical and OH in cis or trans-HONO. As shown in Fig. 3, there are six pathways can lead to products P2 H2O2 + NO. In the six pathways, the most two possible pathways are the Path11 and Path12 mentioned above. One can find that, the rate-determining energy barriers, TSR 0 - P2 (28.00) and TSR-P2 (36.44), are higher than those in the hydrogen abstraction path-
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ways, and consequently, products P2 should be the minor product compared with P1 in the title reaction for kinetics and thermodynamics points of view in all temperature ranges. 3.1.3. Addition–isomerization–elimination mechanism As shown in Fig. 3, the rest reaction channels belong to addition–isomerization–elimination mechanism, which is more complex than previous report [11a]. For discussion concision, we just focus our attention on the minimum energy pathways (MEP) of products P1, P3 7, P9 and cis–trans isomerization reactions. 3.1.3.1. P1 NO2 + H2O formation. There are other four pathways can lead to products P1. The most feasible pathway is an addition–H-shift–elimination reaction process. It is listed below: Path14 R 0 P1 (6): TS1a- C6 fi C6 fi P1
R 0 fi C1 fi TSC1-1a fi 1a fi
3.1.3.2. P3 7, P9 formation. In these reaction steps the high relative energy of rate-determining transition states may rule out their significance in atmospheric chemistry in low temperatures, and even in higher temperature ranges. For conciseness, we only list them as follows: Path15 R 0 P3 (1): R 0 fi TSR 0 -7 fi 7 fi TS7- 13 fi 13 fi TS13-P3 fi P3 HNOO + OH Path16 RP4 (1): R fi C5 fi TSC5- 2 fi 2 fi TS2-P4 fi P4. HNO3 + H Path17 R 0 P5 (1): R 0 fi TSR 0 -7 fi 7 fi TS7- 8 fi 8 fi TS8-12 fi 12 fi TS12-C9 fi C9 fi P5 HOO + HNO Path18 R 0 P6 (1): R 0 fi TSR 0 -5 fi 5 fi TS5- P6 fi P6 OON + H2O Path19 RP7 (1): R fi TSR-3 fi 3 fi TS3-C10 fi C10 fi P7 HO(H)O + NO Path20 RP9 (1): R fi TSR-3 fi 3 fi TS3-C12 fi C12 fi P9 HOO + HON We should mention that, as shown in Fig. 3, four isomers 3 (31.31), 5 (32.77), and 7 (33.95) formation processes are in low potential well at the CCSD(T)//aug-cc-pVDZ level of theory. This implies that such isomers might be short-lived and not important on the PES [53] and can be safely ruled out. On the other hand, one can find that the oxygen of OH radical attack on terminal O-atom of HONO mechanisms processes involve negative barriers at CCSD(T)//aug-cc-pVDZ levels. In contrast, the IRCMax method consistently gives positive barriers that differ from the CCSD(T)//aug-cc-pVDZ levels by 2–3 kcal/mol with sizable displacements along the reaction path (0.7 < S < 1.0,). We expect that large errors are due to poor transition state geometries and positions along the IRCs at a low level of theory and thus have considerable effects on the calculated energy barriers.
3.1.3.3. The OH radical exchange reactions. There are other two reaction pathways for the OH radical exchanges with the OH group in HONO except for Path13 which is mentioned in the section 3.1.2. One can find, from Fig. 3, that both the barriers involved in the OH exchange pathways are moderate (DE<10; DG < 18 kcal/mol) and the exchange processes are not endothermic. Therefore, both kinetic and thermodynamic considerations support the viability of such channels. We simply list them below for concision: Path21 RR 0 (1): R fi C4 fi TSC4- 1b fi 1b fi TS1bC1 0 fi C1 0 fi R 0 Path22 R 0 R (1): R 0 fi C1 fi TSC1- 1a fi 1a fi TS1a-C4 0 fi C4 0 fi R
3.2. Comparison with experiments Recently, Robert A. Fifer [29] carried out the kinetic study for the title reaction. The reaction heats of formation calculated are in good agreement with that obtained experimentally. In the experiment, they suggested that the NO2 + H2O formation is the most important channel for this chemical reaction. On the other hand, the speculated that cis-HONO is chiefly responsible for the abstraction reaction. However, they did not provide unambiguous information on the H-atom abstraction channel. Furthermore, they did not give any information about the formation of H2O2 + NO, which may be found in high temperature ranges. In our calculations, it is shown that the most feasible pathway Path1 is the OH radical indirectly abstracts the H-atom of cis-HONO leading to the products P1 NO2 + H2O. Since the energies of the complexes, transition state and products in this pathway are lower than that of the reactants, the rate of this pathway should be very fast and have negative temperature dependence. This is qualitatively consistent with the experimental results [29,35]. The other three pathways Path2, Path4 and Path6 may play a part role at room temperature. Finally, it should be pointed out that in our calculations that the products H2O2 + NO might be found in the title reaction system in high temperature conditions. 3.3. Comparison with previous theory study The title reaction has been reported by Xia etc. [11a]. Geometry optimization and vibrational frequency calculations were carried out at the density functional B3LYP method with the 6-311G(d) basis set. There are some differences between the two studies, the details as show below. In our studies we find that the hydrogen abstraction reaction mechanism is more complex than previous thought. The reaction mechanisms involved are: Singlebarrier indirect hydrogen abstraction reaction mechanism, double-barriers indirect hydrogen abstraction reaction mechanism, and direct hydrogen abstraction reaction
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mechanism. We present twelve reaction pathways through which the reaction can lead to products P1 NO2 + H2O, including the pathways that the hydrogen of the OH radical attack on hydroxyl-oxygen of HONO, which have not been reported previously. Furthermore, we also find the pathway of the hydrogen of the OH attack on nitrogen of HONO and the pathway of the hydrogen of the OH radical attack on the terminal oxygen at the same time the oxygen of the OH radical attack on nitrogen of HONO. In addition, several more complex pathways are also revealed, namely, the oxygen of OH radical attack on the terminal oxygen of HONO. Considering that the activation barriers and/or the relative energies of products are high, the title reaction may proceed under combustion environment, where the temperature is very high (>1000 K), and these pathways may play a part role and should be helpful for the reverse reactions. 3.4. Comparison with the reactions of H + HONO and F + HONO To deepen understand the reaction mechanism of OH + HONO, it is worthwhile to compare the title reaction with analogous reaction H + HONO, and F + HONO which have been studied theoretically [16,18]. Theoretical calculations show that they exist in similarity and discrepancy. The similarity of the three reactions is that the cis-HONO isomer is apparently more reactive, as has been concluded in the NH2 + HONO reaction [54]. On the other hand, the discrepancy lies in the competition abilities of pathways. For H + HONO reaction, the direct H-abstraction reactions is a minor process and that the indirect metathetical reactions taking place by H-atom addition to the Natom as well as to the terminal O-atom, producing HNO + OH and NO + H2O, are the dominant reaction channels. For the reaction of F + HONO, the most feasible reaction pathway is the direct H-abstraction reaction lead to NO2 + HF. For OH + HONO, the most feasible reaction pathways are the indirect H-abstraction reactions leading to P1 NO2 + H2O, which is analogous with the reaction F + HONO. The other feasible pathways are the oxygen of the OH radical attack on nitrogen of HONO proceeding the direct hydroxyl exchange and the oxygen of the OH radical attack on hydroxyloxygen of HONO transforming to educts P2 H2O2 + NO. It is obvious that the reaction of OH + HONO involves all the main features of H + HONO and F + HONO reactions. However, from the discussions above, it is obvious that the reaction mechanisms of OH + HONO are more complicated than those of H + HONO and F + HONO. 4. Conclusions A detailed theoretical survey on the complicated doublet PESs of the reaction of OH radical with nitrous acid HONO
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has been performed at the CCSD(T)//UMP2 levels of theory. Three different mechanisms and four major pathways are revealed in the present study. In the hydrogen abstraction reaction mechanism, the major pathways are Path1, Path2, Path4 and Path6 on the doublet PES. The pathway Path1 is the most competitive while pathway Path6 is least competitive among the four major pathways. In Path1, the indirect hydrogen abstraction process starts from the OH radical attacking on the hydrogen of cis-HONO, forming a 6-member-ring complex C2 with no energy barrier. And then C2 can transform to a weakly bound complex C6 via an H-atom abstraction transition state TSabsC2-C6I. Subsequently, C6 transforms to the separated products P1 H2O + NO2 directly. Since the relative energies of the complexes, transition state and products in this pathway lie lower than that of the reactants, the rate of this pathway should be very fast and have negative temperature dependence. On the other hand, our calculation results show that, in terms of potential energy surface, the title reaction involves all the main features of H + HONO and F + HONO reactions. However, the mechanisms of OH radical with HONO are rather more complicated than those two reactions. According to our results, the presence of prereactive complexes indicates that the simple hydrogen abstraction in the radical reaction with carbon-free compounds should be more complex. Based on the analysis of the energetics of all channels through which the abstraction, association, and addition reactions proceed, we expect that the reaction varieties should be considerable complicated for the title reaction under different experimental conditions. Therefore, future experimental studies on the title reaction are highly desirable under various pressures and temperatures. All in all, the present work will provide useful information for understanding the processes of OH radical reactions with other carbon-free compounds. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 20073014, 20643004 and 20103003), Excellent Young Teacher Foundation of the Ministry of Education of China, Excellent Young Foundation of Jilin Province and Technology Development Project of Jilin Province (No. 20050906-6). Appendix A. Supplementary data Table S1 gives the energies, enthalpies, and Gibbs free energies. Table S2 shows the values of the spin contamination before and after annihilation. Fig. S1 gives schematic potential energy surfaces of the associative reaction mechanism, and the addition-isomerizationelimination channels. This material is available free of charge via the Internet at E-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.theochem.2007.08.037.
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OH + HONO reaction: A theoretical study De-Quan Wang, Ji-Lai Li, Xu-Ri Huang *, Cai-Yun Geng, Chia-Chung Sun State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China Received 19 June 2007; received in revised form 24 August 2007; accepted 24 August 2007 Available online 11 September 2007
Abstract The reaction of OH radical with nitrous acid HONO is investigated by ab initio quantum chemistry methods. The doublet potential energy surfaces are calculated at the CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) levels. Various possible reaction pathways are considered. Among them, the most feasible pathway should be the OH radical attacking on the hydrogen of cis-HONO to form a 6-member-ring complex C2 barrierlessly, followed by the indirect hydrogen abstraction transition state TSabsC2-C6I to form a weakly bound complex C6, giving rise to the educts P1 H2O + NO2. Because all of the complexes, transition state, and products involved in the feasible pathway lie below the reactants, the title reaction is expected to be rapid, which in good agreement experiment. The present study may be helpful for probing the mechanisms of the HONO reactions and understanding the atmospheric chemistry. 2007 Elsevier B.V. All rights reserved. Keywords: Reaction mechanism; Potential energy surface; OH radical; HONO
1. Introduction Radical-molecule reactions play important roles in both combustion and interstellar processes [1]. Reactions with small or even zero barriers are of particular interest [2–6], especially in interstellar space where the temperature is low. Under these conditions, low-barrier reactions easily deplete molecules and yield new products. The reaction of hydroxyl radical with nitrous acid is very important to the chemistry of troposphere where HONO may be formed by lighting or chemical reactions in the polluted environment. Surprisingly, data on non-reactive uptake of nitrous acid on ice are scarce, given the importance of HONO for atmospheric and especially polar snow chemistry [7–10]. On the other hand, the reaction is also pivotal to our understanding of high-temperature combustion of nitrate esters, nitramines and energetic materials containing [H, N, O]-species [11]. It has been long known that nitrous acid is a key reactive intermediate during the course of chemical
*
Corresponding author. Fax: +86 431 8894 5942. E-mail address: [email protected] (X.-R. Huang).
0166-1280/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.08.037
combustion reactions of energetic materials such as nitramines propellants [12–15]. On account of its relevancy in combustion and atmospheric chemistry, kinetic and mechanistic information on H, O, NO2, O3, HCl, F, HF, HNO and NH3 initiated reaction with nitrous acid has been subject to numerous experimental and theoretical studies [16– 21]. The most important chemical cleaning agent of the atmosphere is the hydroxyl radical, OH [22,23]. It determines the oxidizing power of the atmosphere, and thereby controls the removal of nearly all gaseous atmospheric pollutants [24,25]. The atmospheric supply of OH is limited, however, and could be overcome by consumption due to increasing pollution and climate change [26,27], with detrimental feedback effects. Recently Rohrer etc. reported that the concentration of OH is linear dependence on solar ultraviolet radiation [28]. Literaturely, there have been limited but conflicting kinetic data [29–38] for the OH + HONO reaction, which has been invariably assumed to occur by the abstraction reaction: OH þ HNO2 ! H2 O þ NO2
DH ¼ 41 kcal=mol
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In the temperature range of interest to atmospheric chemistry, Cox and co-workers reported the rate constant at ambient temperature to fall within the range of (1– 4) · 1012 cm3 mol1 s1 with small positive activation energy, 3.2 kJ/mol, and independent of pressure [30–34]. Recently, Burkholder et al. [35] determined the rate constant by LIF (laser-induced fluorescence) for OH detection and reported k1 = (2.8 ± 1.3) · 1012 exp [(+260 ± 140)/ T] cm3 mol1 s1 for the temperature range 298–373 K. The activation energy for the reaction was found to be 2.2 kJ/mol, clearly at odds with that of Jenkin and Cox mentioned above. Furthermore, another two conflicting rate constant expressions have been recommended: k1 = 3.76 · 1012 T1.0 exp (68/T) cm3 mol1 s1 for 300– 2500 K by Tsang and Herron [36] for high-temperature combustion modeling applications, k1 = 1.63 · 1012 exp (+260/T) cm3 mol1 s1 at 290–380 K by Atkinson et al. [37] for atmospheric modeling applications. On the other hand, a theoretical study [11a] on the title reaction was carried out, and the equilibrium geometries and transition structures were calculated using density functional theory (DFT) method. As we know, different in geometry will lead to different reaction mechanisms and extreme care should be taken to choose an appropriate theoretical level for investigation of reactions involving radicals [39]. Thus, it is necessary to reexamine the title reaction with other theoretical method to provide theoretical knowledge accurate enough to be useful for further kinetic and thermodynamic study on radical reactions. Some conclusions that are made in this work may be helpful for further theoretical and experimental studies of this title reaction. In the title reaction there are pre-reactive complexes. Six pre-reactive complexes are successfully located on the doublet PESs. All the six structures with no imaginary frequencies (minima) were located at all levels of theory. As shown in Fig. 1, most of the addition and the H-abstraction reactions proceed via complexes in the title reaction. Another theoretical investigation of the isoelectronic system C2H2 + O [40], shows that a collinear hydrogenbonded complex C2H OH (productlike) exist on the PES, which is viewed as a direct precursor for the proton transfer. While in another isoelectronic system C2H2 + OH, there is no pre-reactive complex located on the H-abstraction path, but on the addition path [41]. On the other hand, all the ab initio calculations show that the approach of H/O atom of OH radical to HONO generates six different initial pre-reactive complexes, depending on the details of the approach geometry. 2. Computational methods All the calculations were carried out on the SGI O3800 servers using the GAUSSIAN 98 and GAUSSIAN 03 program package [42]. Geometries of the reactants, products, intermediates and transition states (TSs) have been optimized with the unrestricted Møller–Plesset secondorder perturbation UMP2 (FULL) [43] method using
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the 6-311G(d,p), and 6-311++G(d,p) basis sets. The presence of diffuse functions in the basis set allows for an appropriate representation of the dispersion forces that should play an important role in the stabilization of the weakly bound structures considered in this work [44]. Vibrational frequencies, also calculated at the same level of theory, have been used to characterize stationary points, zero-point energy (ZPE) corrections calculations. The number of imaginary frequencies for intermediates and transition states are 0 and 1, respectively. The ZPE and vibrational frequencies were scaled by a factor of 0.95 for anharmonicity correction [45]. To confirm that the transition states connect between designated intermediates, intrinsic reaction coordinate (IRC) [46] calculations were performed at the UMP2/6-311++G(d,p) level of theory. For the purpose of obtaining more reliable energies of various structures, the coupled-cluster CCSD(T) method with Dunning’s correlation-consistent augmented aug-ccpVDZ basis set [47,48] was used. The UMP2/6311++G(d,p) optimized geometries were used for the single-point coupled cluster calculations without reoptimization at the CCSD(T)/aug-cc-pVDZ. The major problem in the application of unrestricted single determinant reference wave function is that of contamination with higher spin states. The severe spin contamination could lead to a deteriorated estimation of the barrier height [49,50]. We have examined the spin contamination before and after annihilation for the radical species and transition states involved in the OH + HONO reaction. For doublet systems, the expectation values of ÆS2æ after annihilation range from 0.75 to 0.80 (the exact value for a pure doublet is 0.75) except for one complex and six unimportant transition states on the potential energy surface (0.80–0.87). This suggests that the wave function is not severely contaminated by states of higher multiplicity [51,52].Therefore, we expect that the CCSD(T)//MP2 calculations reported in this work are, in this regard, reliable. The thermodynamic functions (DH and DG) are estimated within the ideal gas, rigid-rotor, and harmonic oscillator approximations. A temperature of 298.15 K and a pressure of 1 atm were assumed. 3. Results and discussion For the present H2NO3 system, there are enantiomorphs. For convenient and clear discussion, we only present moieties of reaction channels that have enantiomorphic varieties and some important nantiomorphous structures. The structures of the reactants, complexes, intermediate isomers, transition states and products are depicted in Fig. 1. By means of the interrelation among the reactants, isomers, transition states, and products as well as the corresponding relative energies, the schematic profiles of the doublet potential energy surfaces are depicted as shown in Figs. 2, 3 and Fig. S3. The calculated
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Fig. 1. Optimized geometries of the reactants, the complexes, the doublet intermediate isomers, the products and the transition states for the OH + HONO reaction. Bond distances are in angstroms, and angles are in degrees. Numbers in roman type show the structures at the UMP2/6311++G(d,p) level of theory. Numbers in italics show the structures at the UMP2/6-311G(d,p) level of theory. Numbers in bold show the structures at the Max{CCSD(T)}//IRC{UMP2} + ZPE level of theory. Symbols with asterisks indicate enantiomorphic varieties corresponding to the same symbols.
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Fig. 1 (continued)
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Fig. 1 (continued)
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Fig. 1 (continued)
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Fig. 1 (continued)
energetic data (DE, DH, and DG) of various complexes, isomers, transition states and products are listed in Table S1. For convenient discussion, the energy of OH + cis-HONO is set at zero as a reference for other species. 3.1. Analysis of reaction mechanism of OH radical with HONO As the OH + HONO reaction can have either an abstraction or an addition mechanism, three distinguishable types of initial attacks have been revealed for the radical-molecule reaction OH + HONO, namely, the picking-up on hydrogen (direct/indirect hydrogen abstraction), hydroxyl (direction hydroxyl abstraction), and the attack on the O-atom and
N-atom of HONO (addition–isomerization–elimination). A detailed discussion on the title reaction mechanism is given as follows. 3.1.1. Hydrogen abstraction reaction mechanism Three kinds of hydrogen abstraction reactions can take place in the title reaction, viz. the oxygen of the OH radical picking-up hydrogen of HONO directly or indirectly (single-barrier and double-barriers), and the O/N-atom of the HONO indirect picking-up hydrogen of OH radical. 3.1.1.1. The oxygen of OH radical attack on the hydrogen of HONO. Indirect hydrogen abstraction reaction mechanism I
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Fig. 2. Schematic potential energy surfaces of the hydrogen abstraction channels in the OH + HONO reaction. Numbers in roman type are relative energies at the CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) + ZPE level of theory. Relative energy of complex C7 is calculated at the CCSD(T)/augcc-pVDZ//UMP2/6-311G(d,p) + ZPE level of theory.
(single-barrier hydrogen abstraction). The first step of this mechanism involves the O atom of OH radical indirectly attacking the H-atom of HONO. For cis-HONO, there are three pathways of the indirect hydrogen abstraction reactions, and for trans-HONO, two. These pathways can be expressed as follows: Path1 RP1 (1): R (cis-HONO + OH) fi C2 fi TSabsC2-C6I fi C6 fi P1 NO2 + H2O Path2 RP1 (2): R fi C2 fi TSabsC2- C6II fi C6 fi P1
Path3 RR (1): R fi C2 fi TSabsC2- C2 0 fi C2 0 fi R Path4 R 0 P1 (1): R 0 (OH + trans-HONO) fi C1 fi TSabsC1- C6I fi C6 fi P1 Path5 R 0 P1 (2): R 0 fi C1 fi TSabsC1- C6II fi C6 fi P1 As shown in Fig. 2, the first two indirect hydrogen abstraction processes start from the OH radical attacking on the hydrogen of cis-HONO, forming a 6-member-ring complex C2 (3.94 kcal/mol) barrierlessly. And then C2 can transform to a weakly bound complex C6 (34.32)
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Fig. 3. Selected schematic potential energy surfaces of the associative reaction mechanism, and the important addition–isomerization–elimination channels in the OH + HONO reaction. Numbers in roman type are relative energies at the CCSD(T)/aug-cc-pVDZ//UMP2/6-311++G(d,p) + ZPE level of theory. Numbers in square brackets are values at the Max-{CCSD(T)}//IRC{UMP2} + ZPE level of theory.
via two H-atom abstraction transition states, TSabsC2-C6I (2.07) and TSabsC2-C6II (1.32), respectively. Subsequently, C6 transforms to the educts P1 H2O + NO2. Since all of the complexes, transition state and products lie below the reactants, the rate of this pathway should be very fast even in low-temperature ranges. On the other hand, considering that the rate-determining transition state TSabsC2C6II lies 1.32 kcal/mol above the reactants in Path2, we can safely draw the conclusion that the reaction pathway Path2 is also feasible on the doublet potential energy surface at room-temperature. However, the higher rate-determining energy barrier in Path2 makes Path2 less competitive than Path1.
Pathway Path3 is indeed an H-atom exchange process between OH radical and cis-HONO. In pathway Path3, complex C2 and C2 0 can be formed barrierless with the association of hydroxyl and cis-HONO. Complex C2 (C2 0 ) can transform to C2 0 (C2) via TSabsC2-C2 0 , with an 18.14 kcal/mol energy barrier. Both complexes, C2 and C2 0 located by IRC calculations, are essentially the mirror image of each other, having 3.94 kcal/mol binding energies with Cs symmetry. Furthermore, a similar exchange process involving the trans-HONO does not exist. The OH radical and trans-HONO can also form a 5member-ring complex C1 in pathways Path4 and Path5.
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Complex C1 has a slightly larger binding energy (4.56) than its cis-counterpart C2. For products formation, however, C1 has to overcome energy barriers TSabsC1-C6I (1.67 kcal/mol), or TSabsC1-C6II (15.40 kcal/mol) to reach C6, in Path4 and Path5, respectively. Thus the indirect hydrogen abstraction process of trans-HONO by OH may proceed through Path4. Indirect hydrogen abstraction reaction mechanism II (double-barriers hydrogen abstraction). There exists a remarkable difference between the trans and cis reaction processes, that is, for the OH radical picking-up the Hatom of trans-HONO process, there is a double-barriers hydrogen abstraction reaction mechanism, viz. indirect hydrogen abstraction reaction mechanism II. To our knowledge, it is the first time that the existence of double barriers in hydrogen abstraction reactions has been reported. The pathway Path7 takes place via the complex C1, transition state TSabsC1-I0, isomer I0, transition state TSabsI0-C7 and the complex C7, giving rise to the separated H2O + NO2 products with the energies of 4.56, 3.56, 4.04, 3.18 and 34.54 kcal/mol, respectively. As shown in Fig. 2, the rate-determining transition state TSabsI0-C7 in Path6 lies somewhat higher than that of Path1, Path2 and Path4, which leads us to the fact that the Path6 is less competitive than those three pathways, but it could also occur at room-temperature. It can be expressed as follows: Path6 R 0 P1 (3): R 0 fi C1 fi TSabsC1- I0 fi I0 fi TSabsI0-C7 fi C7 fi P1 Direct hydrogen abstraction reaction mechanism. Among all the OH radical attack on the H-atom of HONO pathways, only one direct hydrogen abstraction reaction path is revealed in the present investigation. We express it below for discussion concision: Path7 RP1 (3): R fi TSabsR-C7 fi C7 fi P1 In this pathway, the rate-determining hydrogen abstract transition state TSabsR-C7 lies 23.19 kcal/mol higher than the reactants, the rate of this pathway should be very slow at moderate conditions. Consequently, the large barrier also prevents it from competing effectively with the reaction pathways Path1 and Path2. We should mention herein that we could not find the complex C7 at the UMP2/6311++G(d,p) level. To rationalize this situation, we turn to the flatness of the PES in the regions of larger intermolecular distances as can be seen from Fig. 1. To briefly summarize, the pathway Path1 is the most competitive pathway among all of the seven pathways. The pathway Path2, Path4 and path6 may be less competitive dynamically and play a part role at room temperature. 3.1.1.2. The hydrogen of OH radical attack on the O/N-atom of HONO. Our results show that there are additional three hydrogen abstraction reaction channels, namely, the
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hydrogen of OH radical attacking on hydroxyl-oxygen, and nitrogen-atom of HONO. They are listed as follows: Path8 RP1 (4): R fi C4 fi TSabsC4-C6 fi C6 fi P1 Path9 R 0 P1 (5): R 0 fi C3 fi TSabsC3-C6 fi C6 fi P1 Path10 RP8 (4): R fi C5 fi TSabsC5-P8 fi P8 HON(H)O + O As shown in Fig. 1, transition states TSabsC4-C6 (43.05) and TSabsC3-C6 (40.39) possess similar 4-member-ring structures in Path8 and Path9. In these two reaction pathways, IRC calculations show that, the hydrogen of OH radical approaches the hydroxyl-oxygen of HONO firstly, which makes the bond of HO NO broken, and then, the O H bond of OH radical ruptured. Secondly, the O-atom attacks the N-atom of NO, forming the complex C6, leading to P1 eventually. On the other hand, the hydrogen of OH radical can also attack on the nitrogen of cis-HONO, leading to the production of P8 HON(H)O + O through Path10. Since the rate-determining energy barrier in Path10 is considerably high (83.09), we can safely rule it out from the doublet PES in low-temperature ranges. However, in combustion environment, where the temperature is very high (>1000 K), pathways Path8 and Path9, may play an important role. 3.1.2. Associative reaction mechanism There are three reaction channels in the associative reaction mechanism, that is, Path11 13, in which three associative transition structures are successfully established. The paths are listed below: Path11 R 0 P2 (1): R 0 fi TSR 0 -P2 fi P2 H2O2 + NO Path12 RP2 (1): R fi TSR-P2 fi P2 Path13 RR 0 (3): R fi C4 fi TSC4-C1 0 fi C1 0 fi R 0 As shown in Fig. 1, the main feature of the transition state TSR 0 -P2 in the path11 is: It has an imaginary frequency of 228i cm1 and the animation of imaginary frequency corresponds to the movement of OH and NO group, when OH moves toward the O-atom of hydroxyl in trans-HONO, the NO group would be away from it, that is to say, the formation of the O–O bond is concomitant with the breaking of the O–N bond. A corollary, the formation of the O–N bond would be concomitant with the breaking of the O–O bond. The transition states TSR-P2 and TSC4-C1 0 own the similar feature as well except the different moving groups. That is, TSR-P2 corresponds to the movement of OH radical and NO in cis-HONO, which is reported for the first time, while TSC4-C1 0 corresponds to the movement of the OH radical and OH in cis or trans-HONO. As shown in Fig. 3, there are six pathways can lead to products P2 H2O2 + NO. In the six pathways, the most two possible pathways are the Path11 and Path12 mentioned above. One can find that, the rate-determining energy barriers, TSR 0 - P2 (28.00) and TSR-P2 (36.44), are higher than those in the hydrogen abstraction path-
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ways, and consequently, products P2 should be the minor product compared with P1 in the title reaction for kinetics and thermodynamics points of view in all temperature ranges. 3.1.3. Addition–isomerization–elimination mechanism As shown in Fig. 3, the rest reaction channels belong to addition–isomerization–elimination mechanism, which is more complex than previous report [11a]. For discussion concision, we just focus our attention on the minimum energy pathways (MEP) of products P1, P3 7, P9 and cis–trans isomerization reactions. 3.1.3.1. P1 NO2 + H2O formation. There are other four pathways can lead to products P1. The most feasible pathway is an addition–H-shift–elimination reaction process. It is listed below: Path14 R 0 P1 (6): TS1a- C6 fi C6 fi P1
R 0 fi C1 fi TSC1-1a fi 1a fi
3.1.3.2. P3 7, P9 formation. In these reaction steps the high relative energy of rate-determining transition states may rule out their significance in atmospheric chemistry in low temperatures, and even in higher temperature ranges. For conciseness, we only list them as follows: Path15 R 0 P3 (1): R 0 fi TSR 0 -7 fi 7 fi TS7- 13 fi 13 fi TS13-P3 fi P3 HNOO + OH Path16 RP4 (1): R fi C5 fi TSC5- 2 fi 2 fi TS2-P4 fi P4. HNO3 + H Path17 R 0 P5 (1): R 0 fi TSR 0 -7 fi 7 fi TS7- 8 fi 8 fi TS8-12 fi 12 fi TS12-C9 fi C9 fi P5 HOO + HNO Path18 R 0 P6 (1): R 0 fi TSR 0 -5 fi 5 fi TS5- P6 fi P6 OON + H2O Path19 RP7 (1): R fi TSR-3 fi 3 fi TS3-C10 fi C10 fi P7 HO(H)O + NO Path20 RP9 (1): R fi TSR-3 fi 3 fi TS3-C12 fi C12 fi P9 HOO + HON We should mention that, as shown in Fig. 3, four isomers 3 (31.31), 5 (32.77), and 7 (33.95) formation processes are in low potential well at the CCSD(T)//aug-cc-pVDZ level of theory. This implies that such isomers might be short-lived and not important on the PES [53] and can be safely ruled out. On the other hand, one can find that the oxygen of OH radical attack on terminal O-atom of HONO mechanisms processes involve negative barriers at CCSD(T)//aug-cc-pVDZ levels. In contrast, the IRCMax method consistently gives positive barriers that differ from the CCSD(T)//aug-cc-pVDZ levels by 2–3 kcal/mol with sizable displacements along the reaction path (0.7 < S < 1.0,). We expect that large errors are due to poor transition state geometries and positions along the IRCs at a low level of theory and thus have considerable effects on the calculated energy barriers.
3.1.3.3. The OH radical exchange reactions. There are other two reaction pathways for the OH radical exchanges with the OH group in HONO except for Path13 which is mentioned in the section 3.1.2. One can find, from Fig. 3, that both the barriers involved in the OH exchange pathways are moderate (DE<10; DG < 18 kcal/mol) and the exchange processes are not endothermic. Therefore, both kinetic and thermodynamic considerations support the viability of such channels. We simply list them below for concision: Path21 RR 0 (1): R fi C4 fi TSC4- 1b fi 1b fi TS1bC1 0 fi C1 0 fi R 0 Path22 R 0 R (1): R 0 fi C1 fi TSC1- 1a fi 1a fi TS1a-C4 0 fi C4 0 fi R
3.2. Comparison with experiments Recently, Robert A. Fifer [29] carried out the kinetic study for the title reaction. The reaction heats of formation calculated are in good agreement with that obtained experimentally. In the experiment, they suggested that the NO2 + H2O formation is the most important channel for this chemical reaction. On the other hand, the speculated that cis-HONO is chiefly responsible for the abstraction reaction. However, they did not provide unambiguous information on the H-atom abstraction channel. Furthermore, they did not give any information about the formation of H2O2 + NO, which may be found in high temperature ranges. In our calculations, it is shown that the most feasible pathway Path1 is the OH radical indirectly abstracts the H-atom of cis-HONO leading to the products P1 NO2 + H2O. Since the energies of the complexes, transition state and products in this pathway are lower than that of the reactants, the rate of this pathway should be very fast and have negative temperature dependence. This is qualitatively consistent with the experimental results [29,35]. The other three pathways Path2, Path4 and Path6 may play a part role at room temperature. Finally, it should be pointed out that in our calculations that the products H2O2 + NO might be found in the title reaction system in high temperature conditions. 3.3. Comparison with previous theory study The title reaction has been reported by Xia etc. [11a]. Geometry optimization and vibrational frequency calculations were carried out at the density functional B3LYP method with the 6-311G(d) basis set. There are some differences between the two studies, the details as show below. In our studies we find that the hydrogen abstraction reaction mechanism is more complex than previous thought. The reaction mechanisms involved are: Singlebarrier indirect hydrogen abstraction reaction mechanism, double-barriers indirect hydrogen abstraction reaction mechanism, and direct hydrogen abstraction reaction
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mechanism. We present twelve reaction pathways through which the reaction can lead to products P1 NO2 + H2O, including the pathways that the hydrogen of the OH radical attack on hydroxyl-oxygen of HONO, which have not been reported previously. Furthermore, we also find the pathway of the hydrogen of the OH attack on nitrogen of HONO and the pathway of the hydrogen of the OH radical attack on the terminal oxygen at the same time the oxygen of the OH radical attack on nitrogen of HONO. In addition, several more complex pathways are also revealed, namely, the oxygen of OH radical attack on the terminal oxygen of HONO. Considering that the activation barriers and/or the relative energies of products are high, the title reaction may proceed under combustion environment, where the temperature is very high (>1000 K), and these pathways may play a part role and should be helpful for the reverse reactions. 3.4. Comparison with the reactions of H + HONO and F + HONO To deepen understand the reaction mechanism of OH + HONO, it is worthwhile to compare the title reaction with analogous reaction H + HONO, and F + HONO which have been studied theoretically [16,18]. Theoretical calculations show that they exist in similarity and discrepancy. The similarity of the three reactions is that the cis-HONO isomer is apparently more reactive, as has been concluded in the NH2 + HONO reaction [54]. On the other hand, the discrepancy lies in the competition abilities of pathways. For H + HONO reaction, the direct H-abstraction reactions is a minor process and that the indirect metathetical reactions taking place by H-atom addition to the Natom as well as to the terminal O-atom, producing HNO + OH and NO + H2O, are the dominant reaction channels. For the reaction of F + HONO, the most feasible reaction pathway is the direct H-abstraction reaction lead to NO2 + HF. For OH + HONO, the most feasible reaction pathways are the indirect H-abstraction reactions leading to P1 NO2 + H2O, which is analogous with the reaction F + HONO. The other feasible pathways are the oxygen of the OH radical attack on nitrogen of HONO proceeding the direct hydroxyl exchange and the oxygen of the OH radical attack on hydroxyloxygen of HONO transforming to educts P2 H2O2 + NO. It is obvious that the reaction of OH + HONO involves all the main features of H + HONO and F + HONO reactions. However, from the discussions above, it is obvious that the reaction mechanisms of OH + HONO are more complicated than those of H + HONO and F + HONO. 4. Conclusions A detailed theoretical survey on the complicated doublet PESs of the reaction of OH radical with nitrous acid HONO
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has been performed at the CCSD(T)//UMP2 levels of theory. Three different mechanisms and four major pathways are revealed in the present study. In the hydrogen abstraction reaction mechanism, the major pathways are Path1, Path2, Path4 and Path6 on the doublet PES. The pathway Path1 is the most competitive while pathway Path6 is least competitive among the four major pathways. In Path1, the indirect hydrogen abstraction process starts from the OH radical attacking on the hydrogen of cis-HONO, forming a 6-member-ring complex C2 with no energy barrier. And then C2 can transform to a weakly bound complex C6 via an H-atom abstraction transition state TSabsC2-C6I. Subsequently, C6 transforms to the separated products P1 H2O + NO2 directly. Since the relative energies of the complexes, transition state and products in this pathway lie lower than that of the reactants, the rate of this pathway should be very fast and have negative temperature dependence. On the other hand, our calculation results show that, in terms of potential energy surface, the title reaction involves all the main features of H + HONO and F + HONO reactions. However, the mechanisms of OH radical with HONO are rather more complicated than those two reactions. According to our results, the presence of prereactive complexes indicates that the simple hydrogen abstraction in the radical reaction with carbon-free compounds should be more complex. Based on the analysis of the energetics of all channels through which the abstraction, association, and addition reactions proceed, we expect that the reaction varieties should be considerable complicated for the title reaction under different experimental conditions. Therefore, future experimental studies on the title reaction are highly desirable under various pressures and temperatures. All in all, the present work will provide useful information for understanding the processes of OH radical reactions with other carbon-free compounds. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 20073014, 20643004 and 20103003), Excellent Young Teacher Foundation of the Ministry of Education of China, Excellent Young Foundation of Jilin Province and Technology Development Project of Jilin Province (No. 20050906-6). Appendix A. Supplementary data Table S1 gives the energies, enthalpies, and Gibbs free energies. Table S2 shows the values of the spin contamination before and after annihilation. Fig. S1 gives schematic potential energy surfaces of the associative reaction mechanism, and the addition-isomerizationelimination channels. This material is available free of charge via the Internet at E-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.theochem.2007.08.037.
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